1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling termination of current driven circuits, and particularly, to a method of and an apparatus for controlling termination of current driven circuits in general electronic circuits that bidirectionally transmit and receive a current driven signal such as an ECL (Emitter Coupled Logic) signal or a PCML (Pseudo Current Mode Logic) signal.
2. Description of the Related Art
Recently, the scale of integrated circuits is increasing year by year. It is required to reduce the number of I/O pins by sharing the I/O pins. It is also required for circuits employing ECL signals to transfer data at a correct amplitude under dual or multiple termination control.
In the conventional termination circuit for a current driven circuit, a termination circuit is provided only for an ECL circuit serving as a receiver.
This termination circuit has, for example, a pluraity of resistors and a capacitor, and is used to balance itself with an impedance line such as a 75-ohm strip line or a coaxial cable. However, in the prior art, when data are transmitted from one ECL circuit to another ECL circuit through the impedance line in one direction, the termination circuit works only for the receiver ECL circuit.
By the way, recently, an ECL signal has an amplitude of, for example, 1.0 V with a high level H of -0.8 V and a low level L of -1.8 V. The termination circuit regulates the current driven signal, i.e., the receiver ECL circuit so that the amplitude of the signal is maintained at 1.0 V around a threshold voltage of, for example, -1.3 V. To further improve an operation speed, a PCML (Pseudo Current Mode Logic) signal is employable. The PCML signal has two amplitudes 0.4 V and 0.8 V around threshold voltages of +1.6 V and +2.0 V, respectively.
In the ECL circuits employing the PCML signals, there is required to transfer data at a correct amplitude under dual or multiple termination control. The problems of the related art will be explained later in detail with reference to accompanying drawings.